The origin of complexity

Tiny prokaryotes are still the most numerous life form on Earth. The generally larger eukaryotes, as single cells or larger multicellular organisms, evolved later. The origin of eukaryotic life is a source of intense debate.

Eukaryotes contain organelles that perform specific functions, e.g. energy storage is undertaken by mitochondria and movement by cilia or a long flagellum, and reproduction and the storage ofgenetic information within a nucleus (Fig. 15.2b). The presence of a nucleus is one of the defining features of this group of organisms. Some eukaryotes can combine their DNA with that of another individual - that is they can reproduce sexually, increasing their potential genetic variability. In addition, the cells of eukaryotes contain a cytoskeleton, made up of tiny tubes and proteins, which allows the cell to dispense with a rigid outer wall. As a result, eukaryotes can change their shape, even to the extent of surrounding and consuming other cells.

The origin of eukaryotes was almost certainly by a process known as endosymbiosis. Individual prokaryotic organisms were incorporated into eukaryotic cells to produce an interdependent entity whose functions were carried out by specialized organelles derived from the absorbed organisms (Fig. 15.3). The mitochondria and chloroplasts of eukaryotic cells have an independent genetic code, reflecting their origins as independent organisms. In addition, some modern prokaryotes bear a marked resemblance to the organelles of eukaryotic cells.

Most single-celled eukaryotes are relatively large, usually between 10 and 100 |lm in size. As the eukaryotic cell size increases, so its surface area to volume relationship becomes less favorable so that the movement of molecules across the cell membrane by diffusion will not take place quickly enough to supply the cell. Instead, active transport mechanisms are required to import material from outside the cell. The increase in the cell's energy demands that follows from increasing size is satisfied by the development of oxidative energy production. The citric acid cycle (Krebs cycle) produces far more energy than the anaerobic, glycolitic cycle used by primitive prokar-yotes. Eukaryotic diversification could only have occurred after the oceans and the atmosphere had become oxygenated.

It is likely that gaseous oxygen began to accumulate in the Earth's atmosphere around 2 billion years ago. This might be the point where eukaryotes began to be able to spread geographically and to evolve rapidly as they encountered new environments.

The oldest evidence for eukaryotes comes from chemical fossils 2.7 billion years old. The oldest abundant and widely dispersed body fossils are acritarchs (Fig. 15.4). These are thought to be the resting stages of eukaryotic planktic algae, similar to modern dinoflagellates. The oldest acritarchs are 2.5 billion years old, and they appear to have undergone a major radiation at about 1 billion years ago, becoming common in the fossil record at this time.

Simple ancestor, possibly similar to a methane-producing bacterium

Spirochaetes incorporated and produced structures for locomotion

Simple ancestor, possibly similar to a methane-producing bacterium

Spirochaetes incorporated and produced structures for locomotion

Oxygen-metabolizing prokaryote incorporated and formed mitochondria and possibly cell nucleus

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